1. Field of the Invention
This invention relates generally to radiation sensors and more particularly to bolometer type sensors for detecting thermal radiation.
2. Description of Related Art
Bolometers are used and proposed for use in many new applications. The principal application is construction of thermal cameras. Interest in bolometers stems from the fact that their performance has significantly improved, they""re sensitive at much longer wavelengths, and operate at room temperature. Specifically, IR cameras, with large bolometer arrays have achieved a sensitivity, a Noise Equivalent Temperature resolution (NExcex94T) better than ≈0.05K. Such performance is less than that of quantum detectors; however, for many applications it is adequate and cost effective. Improved bolometer performance is achieved primarily through improved thermal isolation, made possible with advances in IC micro-machining technology. The thermal isolation achieved is about an order of magnitude from radiation limited isolation.
Bolometers inherently operate at slower rates than quantum detectors. However, with staring focal plane arrays, the slow speed limitation is alleviated, since the pixel integration times correspond to the frame rate, and is much longer than line times in scanning systems. Thus the main obstacle to making bolometers more sensitive are practical limitations in thermally isolating each pixel element. With improved thermal isolation, the bolometers performance will directly improve and thereby find wider applications, including potential replacement for cryogenic FLIR cameras. With ideal thermal isolation, the anticipated NExcex94T improvement is about an order of magnitude in sensitivity.
Accordingly it is an object of the present invention to provide an improvement in the sensitivity of electromagnetic radiation sensors.
It is a further object of the invention to provide an improvement in sensors for detecting thermal radiation.
It is yet another object of the invention to provide an improvement in bolometers for detecting thermal radiation with increased sensitivity.
And it is yet a further object of the invention to provide a bolometer structure fabricated in silicon which operates in an improved mode for increased sensitivity.
These and other objects are achieved by elecro-thermal feedback for removing thermal conductance between a bolometer pixel and the environment through its mechanical support structure and electrical interconnects, thereby limiting the thermal conductance primarily through photon radiation. Zeroing the thermal conductance associated with the mechanical support structure and electrical interconnects is achieved by electro-thermal feedback that adjusts the temperature of the intermediate stage, and the mechanical support structure, and electrical interconnects to equal the bolometer""s absorber element temperature.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, the detailed description and the specific examples, while indicating the preferred embodiments of the invention are made by way of illustration only, since various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description.
One aspect of the invention is directed to a thermal radiation sensor assembly, comprising: means for providing a heat bath for the assembly; an absorber element for detecting thermal radiation; an intermediate stage for thermally isolating the absorber element from the means for providing a heat bath and including a thermal isolation member having a predetermined thermal conductance, interconnecting the absorber element to the intermediate stage; and an electro-thermal feedback circuit located within the intermediate stage for reducing the thermal conductivity between the absorber element and the means providing a heat bath by causing the temperature of the intermediate stage to converge to the temperature of the absorber element when detecting thermal radiation, effectively causing the thermal conductance of the thermal isolation member to attain a minimum conductance value and thereby improve the sensitivity of the absorber element to the thermal radiation limit. A plurality of these sensors, moreover, are intended for use in an array.
Another aspect of the invention is directed to a method of sensing thermal radiation, comprising the steps of: locating an absorber element above a heat bath member; interconnecting the absorber element and the heat bath member by a thermal isolation member having a predetermined thermal conductance; and, reducing the thermal conductivity between the absorber element and the heat bath member by causing the temperature of the intermediate stage to converge to the temperature of the absorber element when detecting thermal radiation, effectively causing the thermal conductance of the thermal isolation member to attain a minimum conductance value and thereby improve the sensitivity of the absorber element to the thermal radiation limit.